Optical discs such as compact disks (CDs), video compact disks (VCDs) and digital versatile disk (DVDs) can be played by using kinds of recording and reproducing apparatuses. When an optical pickup head of an optical disk drive is used to access information from an optical disc, the light emitted by a light source such as a laser diode is focused by an objective lens of the optical pickup head and then projected on an optical disc. The light reflected by the optical disc is detected by a photodetector and an operation for realizing information read from the disc is then activated consecutively. Referring to FIG. 1, the optical pickup head 10 moves along two main directions, i.e. a direction perpendicular to the disc face, referred as a focusing direction F, and a direction parallel to the disc face, referred as a tracking direction T.
As one skilled in the art knows, focusing errors and tracking errors are usually arisen when a disc is rotating. The focusing errors are generally caused by vibrations of the disc in the focusing direction F, and the tracking errors are caused by eccentricity of the disc in the tracking direction T. FIG. 2 shows a focus control system for correcting these errors mentioned above. The optical pickup head 20 has a photodetector having four sensors (not shown) that detect reflected light beams from the disc and produce sub-beam signals A, B, C and D, respectively. These sub-beam signals are amplified through a pre-amplifier 21 to generate a focusing error signal Fe and a sub-beam addition signal (SBAD). The focusing error signal Fe is substantially a difference between the summation of sub-beam signals A and C and the summation of sub-beam signals B and D, i.e. (A+C)−(B+D). The sub-beam addition signal SBAD is substantially the summation of these sub-beam signals, i.e. (A+B+C+D). The focusing error signal Fe and the sub-beam addition signal SBAD are processed by a digital signal processor (DSP) 22 to generate a focus control signal Foo used by an actuator 23. Therefore, the actuator 23 may provide a moving force for actuating a trace shift of the optical pickup head 20 along the focusing direction F. When the optical pickup head 20 completes required focusing operation, the digital signal processor 22 produces a focusing completion signal Fok to a micro-controller 24.
FIG. 3 is a timing waveform diagram illustrating the corresponding signals processed in the conventional focus control system of FIG. 2, wherein a focusing error signal Fe changes as the focusing control signal Foo rises. The Foo signal is used for a focus adjustment operation, which moves the pickup head downward to a predetermined position (Foo waveform will be in valley) along the focusing direction F and then pulls up the pickup head gradually accompanied by evaluations to focusing error signal Fe. Basically, the value of the focusing error signal Fe will rise in the beginning and then fall to zero (or less) along the time axis (i.e. the horizontal axis of FIG. 3). When the first peak of the focusing error signal Fe falls to zero at t=t1 (i.e. Fe falls from the peak value to zero at the first time), the digital signal processor 22 starts a closed-loop control operation, and the waveform of the focusing control signal Foo is adjusted in response to the focusing error signal Fe. After the closed-loop control operation starts, the waveform of the sub-beam addition signal SBAD, as shown in a solid line, will maintain at a high level 302. When the sub-beam addition signal SBAD up-crosses a threshold value TV, the focus completion signal Fok will be issued at the end of the predetermined period PT and then transmitted to the micro-controller 24 to indicate the completion of the focusing operation.
Unfortunately, the waveform of the sub-beam addition signal SBAD might have deviations from the ideal waveform in FIG. 3. In practice, an unstable waveform may occur due to the variation of discs as shown in FIG. 4, which results in a misevaluation to the focusing completion timing. For example, a noise 40 occurring prior to the closed-loop control operation exceeds the threshold value TV may activate a wrong counting operation for the predetermined time period PT before the concerned peak 41 appears. At the end of the predetermined time period PT, the focusing completion signal Fok will be incorrectly switched to a high level and abnormally indicate a focusing completion operation. Such a mistake is therefore insufficient to provide a good focus control efficacy and becomes problematic in a high-speed optical disc.